Multi-chassis link aggregation (MLAG) is the ability of two or more network elements to act like a single network element when forming link bundles. This allows a host to uplink to two network elements for physical diversity, while still having a single bundle interface to manage. In addition, two network elements can connect to two other network element using MLAG, with all links forwarding.
Since the network element treats each MLAG as a single bundle interface, the network element can associate this MLAG as a nexthop for an address, such as a Media Access Control (MAC) address. The network element uses an address table to store the association between a MAC address, the associated MLAG and the set of MLAG interfaces for this MLAG. Because the network element may know about lots of MAC addresses, this address table can store hundreds of thousands of MAC addresses or even over a million MAC addresses.
A problem can occur if a one or more of the links in the MLAG go down. If this situation occurs, the network element can use a backup set of links for this MLAG, such as the peer link between the other network element configured for this MLAG. In order to use the backup links, the network element needs to update the address table so as to have a current set of MLAG interfaces for the nexthop of the associated address. Because the network element does not a priori which address is associated to which MLAG, the network element needs to inspect and possibly update each and every entry in the address table. With the address table potentially having hundreds of thousands or even over a million entries, there is some time (e.g., several seconds or more) where the addresses are still associated with the downed link. During this updating time, there is the possibility that data destined for one of the addresses with the non-updated MLAG interface will be dropped.